There is a 1M Ohms 'positive feedback resistor' in the 10mA current source below:

enter image description here

What's the purpose of it?


Some people just think it's not a 'feedback' but simply a resistor tied to ground, but why it is draw like a 'feedback' resistor? However, if it's only a resistor tied to ground, then what's its purpose?

Just see another figure below:enter image description here

Here the 10Meg resistor is a 'positive feedback', right?

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    \$\begingroup\$ The only 1 meg resistor I see in that circuit has one end going to ground, no positive feedback there! \$\endgroup\$ – Neil_UK Jun 13 '17 at 6:19
  • \$\begingroup\$ Yes, but it's strange why it is drawn like this? And see the updated question. \$\endgroup\$ – diverger Jun 13 '17 at 6:46
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    \$\begingroup\$ Drawing circuits correctly is a skill not many even know about. It's like Jedi powers. \$\endgroup\$ – Gregory Kornblum Jun 13 '17 at 6:52
  • \$\begingroup\$ If the values were not shown above then I would vote for "hysteresis", but 10M looks too large to me for that matter. \$\endgroup\$ – Rohat Kılıç Jun 13 '17 at 6:57
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    \$\begingroup\$ I know is an old post, but I think it's to protect op-amp from an "open load" in case of a load failure. See tubecad.com/2012/07/blog0238.htm. \$\endgroup\$ – Adam Calvet Bohl Nov 8 '18 at 15:06

The 1M resistor is not a feedback resistor in the first schematic. Its orientation in the schematic at first glance suggests it is feedback related, but it is simply tied to ground. Its purpose is most likely to limit output transients in the event there is no current load present on its output.

In the second schematic, the resistor is for feedback purposes.

  • \$\begingroup\$ Can you explain how it limit the transients? Because 1M is really a large value. \$\endgroup\$ – diverger Jun 13 '17 at 22:55

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